The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with medical magnetic resonance imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention also finds application in conjunction with other types of magnetic resonance imaging systems, magnetic resonance spectroscopy systems, and the like.
In magnetic resonance imaging, linear magnetic field gradients are used for spatial encoding. Gradient coils are used to produce the linear magnetic field gradients. Gradient coils are generally designed to provide an imaging field-of-view (FoV) that is fixed in size. For example, in whole-body applications the gradient coil will typically be designed to produce sufficiently linear or uniform magnetic field gradients over a 50 cm diameter spherical volume (DSV). For a dedicated cardiac scanner, however, the DSV may be 35 cm. For a dedicated head system the linear gradients would typically be designed to produce sufficiently linear magnetic field gradients over a 25 cm DSV. Of course, some models of scanner are designed with slightly larger or smaller DSVs. As the useful DSV is made smaller, the stored energy of the gradient coil is reduced, which allows for higher performance, namely, higher peak gradient strengths and faster gradient coil switching. Outside the substantially linear region of the gradient field (i.e., the xe2x80x9cusefulxe2x80x9d DSV), and to a lesser extent within, the magnetic field gradients produce image distortion. Software-based distortion correction schemes have been developed to correct for non-uniformities within the useful DSV, as well as to expand somewhat the useful imaging FoV beyond the linear region.
In each dedicated case noted above, the gradient coil is generally a unique electromechanical structure and gradient coils with a defined DSV are known and utilized throughout the industry. For example, the most common is a self-shielded symmetric gradient coil design for whole-body imaging applications. Dedicated head and cardiac/head coil designs have emerged to enhance performance (peak strength and switching rate) over a reduced imaging DSV. Generally, body access is desirable for patient comfort reasons, although dedicated head gradient designs continue to be discussed for advanced neuro/brain research applications.
Gradient coils are heavy electromechanical devices, unlike most RF surface coils, which can be easily removed and replaced with different RF coils between imaging procedures (except for body RF transmit coils, which are typically fixed in the imaging system). A gradient coil, due to its high power nature and the high forces created when it is energized, is firmly fixed within an MRI system. As such, a dedicated gradient coil tends to make the MRI system a dedicated imaging system, limiting its scope of clinical application. Thus, accommodating both large and small FoV applications has generally required either separate dedicated machines, which is expensive, or the use of dedicated insertable coils for the smaller volumes, which are heavy and difficult to insert or replace.
More recently, dual or twin gradient designs have been described in the literature that attempt to combine both large volume and high-performance small volume imaging capabilities into a single gradient coil electromechanical package. Katznelson et al., in U.S. Pat. No. 5,736,858, describe a means for providing two gradient coils, which can be configured to allow for two different useful DSVs. Each gradient axis, x, y, and z, has two gradient coil sets. One gradient coil set is designed to produce a linear magnetic field gradient over a first DSV, and a second gradient coil set is designed to produce a second linear magnetic field gradient, such that when the second gradient coil is driven in series with the first gradient coil, there results a second DSV that is larger than the first DSV. In this scheme, the DSV can take two discrete values but is not continuously variable. The first gradient coil has lower stored energy and can be switched faster than the second gradient coil alone or when the two gradient coils are connected in series. In another embodiment, the first coil produces a gradient for use in small FoV applications and the second coil produces a gradient for use in conventional, large FoV applications and a single amplifier means and a switching means allows for one or the other coil to be used separately. In the preferred embodiment, the first coil is used for fast-switching, small FoV imaging and both coils together are used for larger FoV imaging and/or to produce very high gradient strengths, which may find use in diffusion imaging applications. A key point is that each coil is designed so as to produce, alone or in combination, a linear gradient magnetic field over one of two possible imaging DSVs. In the preferred embodiment the two coils are used together (in series) to produce a relatively large DSV. In the alternate embodiment, each coil can be used individually to create reasonably non-distorted magnetic resonance images over two differently sized DSVs. Each coil is self-shielded or actively-shielded in design to minimize eddy current effects. A drawback of this approach is that two full-power gradient coils are layered within one electromechanical assembly. This consumes a great deal of radial space, particularly when the two coils occupy different radial positions within the electromechanical structure. Since the delivered power increases with R5, even a small increase in gradient coil diameter has significant power ramifications. Also, cooling of the two coils becomes an issue, as does the ability to fit in other components such as passive and electrical shim coils.
It has also been proposed by Kimmlingen et al. (xe2x80x9cGradient system with continuously variable field characteristics,xe2x80x9d ISMRM 2000 (April, 2000, Denver meeting)) to take a standard whole body coil with a large field of view and identify a subset of the primary coil windings that would produce a linear gradient in a smaller FoV, but with comparable (about 20% less) peak gradient strength and substantially lower inductance (about 45% less), allowing for faster gradient switching. A generally corresponding subset of the shield was also selected analogously. A switching means or a dual amplifier design, to feed both coil sections separately, would be provided such that either the subset or all of the windings could be utilized, and the amount of current to subset or other windings could be adjustable, depending on the size of the FoV. The primary advantage is that the primary and shield coils occupy the same radial position with the normal six layers, making cooling and construction easier and more cost-effective. A disadvantage of this approach is that when some of the coil windings were taken away to provide for the smaller FoV, some gradient strength was lost. Another disadvantage is that shielding is compromised since only the combined coils were optimally shielded, leading to increased eddy current effects.
Petropoulos, in U.S. Pat. No. 6,049,207, describes a dual gradient coil assembly with two primary coils and one common shield coil. Each primary coil produces a linear magnetic field gradient over differently sized DSVs when operated with the common shield coil. The residual eddy current effects are not equal for the two coils; one inevitably is better than the other. However, this is minimized by constraining each continuous current primary coil and common shield coil combination to have an integer number of turns before discretization. The approach of having one common shield does save some radial space for manufacture. However, two high power primary coils are still required.
The present invention contemplates a new and improved gradient coil system which provides a selectively or continuously variable imaging field of view, and which overcomes the above-referenced problems and others.
In a first aspect of the present invention, a magnetic resonance imaging apparatus includes a main magnet system for generating a main magnetic field through an examination region, a radio frequency coil disposed adjacent the examination region for transmitting radio frequency signals into the examination region and selectively exciting dipoles disposed therein, and a radio frequency transmitter for driving the radio frequency coil. A receiver receives magnetic resonance signals from resonating dipoles within the examination region and an image processor reconstructs an image representation from the received magnetic resonance signals for display on a human readable display.
The apparatus further includes a gradient coil assembly for generating magnetic field gradients across the main magnetic field. The gradient coil assembly includes a primary gradient coil set disposed about the examination region including an array of conductive coil loops, which are switchable between first and second configurations. In the first configuration, a current flowing thereon generates magnetic field gradients, which are substantially linear over a first useful imaging volume. In the second configuration, a current flowing therein generates magnetic field gradients, which are substantially linear over a second useful imaging volume, which is smaller than the first. The gradient coil assembly further includes a first shielding coil set disposed about the primary coil set. The first shielding coil set includes an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the first configuration. A second shielding coil set disposed about the primary coil set includes an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the second configuration.
In another aspect, a gradient coil assembly for inducing magnetic field gradients across an examination region in a magnetic resonance imaging apparatus is provided. The gradient coil assembly includes a primary gradient coil set disposed about the examination region including an array of conductive coil loops, which are switchable between a first configuration in which a current flowing thereon generates magnetic field gradients, which are substantially linear over a first useful imaging volume and a second configuration in which a current flowing thereon generates magnetic field gradients, which are substantially linear over a second useful imaging volume, which is smaller than the first. A first shielding coil set is disposed about the primary coil set and includes an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the first configuration. A second shielding coil set disposed about the primary coil set includes an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the second configuration.
In still a further aspect, a method of magnetic resonance imaging comprises generating a temporally constant magnetic field through an examination region of a magnetic resonance imaging apparatus, exciting and manipulating magnetic resonance in selected dipoles in the examination region, demodulating magnetic resonance signals received from the examination region, reconstructing the demodulated resonance signals into an image, and, in appropriate time sequence to the above actions, inducing gradient magnetic fields across the temporally constant magnetic field with a gradient coil assembly. The gradient coil assembly comprises a primary gradient coil set disposed about the examination region including an array of conductive coil loops switchable between a first configuration in which a current flowing thereon generates magnetic field gradients which are substantially linear over a first useful imaging volume and a second configuration in which a current flowing therein generates magnetic field gradients which are substantially linear over a second useful imaging volume which is smaller than the first. The gradient coil assembly further includes a first shielding coil set disposed about the primary coil set including an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the first configuration. A second shielding coil set is disposed about the primary coil set and includes an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the second configuration.
In yet a further aspect, in a method of magnetic resonance imaging, a method of producing a magnetic field gradient which is generally linear over a selected imaging volume comprises providing a primary gradient coil having a first configuration to produce a first magnetic field gradient in response to supplying current thereto, the first magnetic field gradient being generally linear over a first useful imaging volume. One or more turns of the primary gradient coil are identified which, when electrically decoupled from the primary coil, reconfigures the primary gradient coil to a second configuration to produce a second magnetic field gradient in response to supplying the first current thereto, the second magnetic field gradient being generally linear over a second useful imaging volume. A first shield coil is configured to substantially shield a fringe field from the primary coil when the primary coil set is operating in the first configuration. A second shield coil is configured to substantially shield a fringe field from the primary coil when the primary coil set is operating in the second configuration.
In still another aspect, a method of designing a gradient coil system for a magnetic resonance imaging system having a variable useful imaging diameter spherical volume, comprises designing a primary gradient coil having a first configuration that produces a first magnetic field gradient that is generally linear over a first imaging volume. One or more turns of the primary gradient coil which, when electrically decoupled from the primary gradient coil, reconfigures the primary gradient coil to a second configuration which produces a second magnetic field gradient that is generally linear over a second imaging volume. A first shield coil is designed that produces a magnetic field which substantially cancels in an area outside a region defined by the shielding coil a first fringe magnetic field generated by the primary gradient coil in one of the first and second configurations. A second shield coil is designed that produces a magnetic field, either alone or in combination with the first shield coil, which substantially cancels in an area outside a region defined by the shielding coil a second fringe magnetic field generated by the primary gradient coil in the other of the first and second configurations.
It will be recognized that the term xe2x80x9csubstantially linear,xe2x80x9d as used herein, is not intended to preclude small non-linearities or non-uniformities in the gradient fields.
One advantage of the present invention is that gradient fields with linear regions of variable spatial extent can be generated to accommodate both large volume imaging applications and small volume imaging applications requiring fast gradient coil switching and high peak gradient strengths.
Another advantage is that the linear region of the gradients can be tailored to the region of interest, thus reducing the potential for peripheral nerve stimulation in an examination subject.
Another advantage is that pluralities of selectable shield coils are provided for efficient shielding for the selected field of view.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.